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CERT Advisory CA-2000-11 MIT Kerberos Vulnerable to Denial-of-Service
Attacks

   Original release date: June 9, 2000
   Last revised: --
   Source: The MIT Kerberos Team, CERT/CC
   
   A complete revision history is at the end of this file.
   
Systems Affected

     * Systems with MIT-derived implementations of the Kerberos 4 KDC
     * Systems with MIT-derived implementations of the Kerberos 5 KDC
       enabled to handle krb4 ticket requests
       
Overview

   The CERT Coordination Center has recently been notified of several
   potential buffer overflow vulnerabilities in the Kerberos
   authentication software. The most severe vulnerability allows remote
   intruders to disrupt normal operations of the Key Distribution Center
   (KDC) if an attacker is able to send malformed requests to a realm's
   key server.
   
   MIT reports that the following versions are vulnerable to one or more
   of these vulnerabilities:
     * MIT Kerberos 5 releases krb5-1.0.x, krb5-1.1, krb5-1.1.1
     * MIT Kerberos 4 patch 10, and probably earlier releases as well
     * KerbNet (Cygnus implementation of Kerberos 5)
     * Cygnus Network Security (CNS -- Cygnus implementation of Kerberos
       4)
       
   Other versions may be affected as well.
   
   The vulnerabilities discussed in this advisory are different than the
   ones discussed in CA-2000-06, Multiple Buffer Overflows in Kerberos
   Authenticated Services. The primary difference is in the impact: the
   new vulnerabilities do not appear to allow remote execution of
   arbitrary code since the buffers being overrun are statically
   declared. In addition, only Kerberos 4 and Kerberos 5 KDC servers that
   can service version 4 ticket requests are affected by the buffer
   overflows discussed here.
   
I. Description

   There are at least five distinct vulnerabilities in various versions
   and implementations of the Kerberos software. All of these
   vulnerabilities may be exploited to effect denial-of-service attacks
   with varying degrees of severity. These vulnerabilities include
     * The buffer used to hold the variable lastrealm in the function
       set_tgtkey() can be owerflowed.
     
     * The buffer used to hold the variable localrealm in the function
       process_v4() can be overflowed.
     
     * The buffer to hold the variable e_msg in the function
       kerb_err_reply() can be overflowed.
     
     * The code that services AUTH_MSG_KDC_REQUESTs does not properly check
       for null-termination.
      
     * Memory that has previously been freed may be improperly freed again,
       possibly resulting in unstable operation.
       
The MIT Kerberos Team Advisory

   The MIT Kerberos Team described these vulnerabilities in more detail
   in an advisory they recently issued. This advisory is available at
   
   http://web.mit.edu/kerberos/www/advisories/krb4kdc.txt
          
II. Impact

   Depending on the version of kerberos, the environment in which its
   running, and the particular vulnerability that is exploited, a remote
   attacker can cause one or more of the following:
     * The KDC to issue invalid tickets for all principles,
     * The KDC to generate a "principal unknown" error, or
     * The KDC process to crash.
       
   Any new authentications to kerberized services will not be possible
   until the KDC is restarted. Note that this implies that operation of
   "kerberized" services will be halted until the KDC is stopped.
   
   It does not appear that any of these vulnerabilities allows the
   execution of code by an intruder.
   
   Additional detail can be found in the MIT advisory.
   
III. Solution

Apply a patch from your vendor

   Appendix A contains information provided by vendors for this advisory.
   We will update the appendix as we receive more information. If you do
   not see your vendor's name, the CERT/CC did not hear from that vendor.
   Please contact your vendor directly.
   
Apply the MIT patches

   If you are running a Kerberos distribution from MIT and can rebuild
   your binaries from source, you can apply the source code patches from
   MIT to correct these problems. These patches are available in the MIT
   Advisory.
   
   If you are running other MIT-derived implementations, you need to
   apply the appropriate vendor patches and recompile the KDC server
   software.
   
Disable Kerberos version 4 authentication in Kerberos version 5 if possible

   As suggested by MIT, krb4 authentication in some daemons can be
   disabled at run time by supplying command-line options to the KDC
   server. Optionally, the krb5 distribution may be compiled with the
   option '--without-krb4' to disable all krb4 ticket handling by
   default.
   
Upgrade to MIT Kerberos 5 version 1.2

   The vulnerabilities described in this advisory will be addressed in
   Kerberos 5 version 1.2. This version will be available from the MIT
   Kerberos web site:
   
   http://web.mit.edu/kerberos/www/
          
Appendix A. Vendor Information

MIT Kerberos

   The MIT Kerberos Team advisory on this topic is available from:
   
   http://web.mit.edu/kerberos/www/advisories/krb4kdc.txt
          
BSDI

   BSDI is working on a patch for this problem and will announce it via
   our normal channels as soon as it is available.
   
NetBSD

   Versions of kerberos which have been integrated into released versions
   of NetBSD and distributed as part of the optional, not-for-export
   "secr" sets are vulnerable to some of the problems cited in the
   advisory. Integration of the fixes is in progress and will be
   announced in a NetBSD security advisory when complete.
   
University of Washington

   [...] we don't distribute client or server binaries with MIT Kerberos
   support.
   
   We distribute source that allows building on UNIX and PC with MIT
   Kerberos. A site which wants to use Kerberos must build our software
   (e.g. Pine, imapd, ipop[23]d) locally in order to use MIT Kerberos.
   
   I did not see anything in this alert that specifically indicates a
   problem for [our] clients or servers. As with all other software built
   with MIT Kerberos, it would be prudent for a site that uses our
   software with MIT Kerberos to rebuild it with the patched version of
   MIT Kerberos.
     _________________________________________________________________
   
   The CERT Coordination Center thanks Tom Yu and the MIT Kerberos Team
   for notifying us about these problem and their help in developing this
   advisory.
     _________________________________________________________________
   
   Jeff Havrilla was the primary author of the CERT/CC portions of this
   document.
   ______________________________________________________________________
   
   This document is available from:
   http://www.cert.org/advisories/CA-2000-11.html
   ______________________________________________________________________
   
CERT/CC Contact Information

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   Revision History
June 9, 2000:  Initial release

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